Ferritic stainless steel and method for manufacturing the same

ABSTRACT

A ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance. The ferritic stainless steel having a chemical composition comprising, by mass %, C: 0.005% or more and 0.035% or less, Si: 0.25% or more and less than 0.40%, Mn: 0.05% or more and 0.35% or less, P: 0.040% or less, S: 0.01% or less, Cr: 15.5% or more and 18.0% or less, Al: 0.001% or more and 0.10% or less, N: 0.01% or more and 0.06% or less, and the balance being Fe and inevitable impurities. Si and Mn satisfy the relational expression 29.5×Si−50×Mn+6≥0, where Si and Mn in the relational expression respectively denote the content, by mass %, of the corresponding chemical element.

TECHNICAL FIELD

This application relates to ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance and a method for manufacturing the steel.

BACKGROUND

Since ferritic stainless steel is inexpensive and excellent in terms of corrosion resistance, it is used for various applications such as building materials, transport machines, home electrical appliances, kitchen appliances, and automobile parts, and its range of application is being further expanded nowadays. A material to be used in such applications is required to have not only excellent corrosion resistance but also sufficient formability (high elongation and a large average Lankford value (hereinafter, also referred to as “average r-value”)) with which it is possible to form the material into a specified shape.

On the other hand, since ferritic stainless steel is used in applications in which the steel is required to have a good surface appearance in many cases, the steel is also required to be excellent in terms of ridging resistance. The term “ridging” refers to surface ruggedness caused by strain due forming work. Since there is a case where a colony, which is a group of crystal grains having similar crystal orientations, is formed when ferritic stainless steel is subjected to casting and/or hot rolling, and since, in the case of a steel sheet in which the colony is retained, there is a large difference in strain between a portion in which the colony is formed and the other portions when forming work is performed, surface ruggedness (ridging) is generated after forming work has been performed. In the case where an excessively large amount of ridging is generated after forming work has been performed, since a polishing process is necessary in order to remove the surface ruggedness, there is a problem of an increase in manufacturing costs of formed products.

In response to the requirements described above, Patent Literature 1 discloses ferritic stainless steel excellent in terms of formability and ridging resistance, the steel having a chemical composition containing, by mass %, C: 0.02% to 0.06%, Si: 1.0% or less, Mn: 1.0% or less, P: 0.05% or less, S: 0.01% or less, Al: 0.005% or less, Ti: 0.005% or less, Cr: 11% to 30%, and Ni: 0.7% or less, in which the relational expressions 0.06≤(C+N)≤0.12, 1≤N/C, and 1.5×10⁻³≤(V×N)≤1.5×10⁻² (C, N, and V respectively denote the mass % of the corresponding chemical elements) are satisfied. However, in Patent Literature 1, it is necessary to perform so-called box annealing (for example, annealing at a temperature of 860° C. for 8 hours) after hot rolling has been performed. Since such box annealing takes about one week including heating and cooling processes, there is a decrease in productivity.

On the other hand, Patent Literature 2 discloses ferritic stainless steel excellent in terms of workability and surface quality, the steel sheet being manufactured by performing hot rolling on steel having a chemical composition containing, by mass %, C: 0.01% to 0.10%, Si: 0.05% to 0.50%, Mn: 0.05% to 1.00%, Ni: 0.01% to 0.50%, Cr: 10% to 20%, Mo: 0.005% to 0.50%, Cu: 0.01% to 0.50%, V: 0.001% to 0.50%, Ti: 0.001% to 0.50%, Al: 0.01% to 0.20%, Nb: 0.001% to 0.50%, N: 0.005% to 0.050%, and B: 0.00010% to 0.00500%, by then performing hot-rolled-sheet annealing on the hot-rolled steel sheet by using a box annealing furnace or a continuous furnace in an AP line (annealing and pickling line) in a temperature range in which a ferrite single phase is formed, and by further performing cold rolling and finish annealing. However, in the case where a box annealing furnace is used, there is a problem of low ductivity as is thecase in Patent Literature 1 described above. In addition, although there is no mention of elongation at all, in the case where hot-rolled-sheet annealing is performed by using a continuous annealing furnace in a temperature range in which a ferrite single phase is formed, since insufficient recrystallization occurs due to a low annealing temperature, there is a case where elongation is lower than in the case where box annealing is performed in a temperature range in which a ferrite single phase is formed. In addition, generally, ins the case of ferritic stainless steel such as that according to Patent Literature 2, since a colony, which is a group of crystal grains having similar crystal orientations, is formed when casting and/or hot rolling is performed, it is not possible to sufficiently break the colony of a ferrite phase in the case where hot-rolled-sheet annealing is performed in a temperature range in which a ferrite single phase is formed Therefore, since the colony is retained in the elongated state in the rolling direction of cold rolling which is performed after hot-rolled-sheet annealing, there is a problem of ridging occurring after forming work has been performed.

As described above, a technique for manufacturing ferritic stainless steel having high formability and ridging resistance with high productivity by using a continuous annealing furnace has not yet been established.

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 3584881 (Domestic Re-publication of PCT International Publication for Patent Application No. WO2000/60134)

PTL 2: Japanese Patent No. 3581801 (Japanese Unexamined Patent Application Publication No. 2001-3143)

SUMMARY Technical Problem

An object of the disclosed embodiments is, by solving the problems described above, to provide ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance and a method for manufacturing the steel.

Here, in the disclosed embodiments, the “sufficient corrosion resistance” refers to a case where a rust area ratio (=(rust area)/(total steel sheet area)×100 [%]) in the surface of a steel sheet is 25% or less the case where a salt spray cyclic corrosion test is performed for 8 cycles on a steel sheet whose surface has been polished by using #600 emery paper and whose end surfaces are sealed in accordance with the prescription in JIB H 8502, where a unit cycle consists of salt spraying (35° C., 5%-NaCl, 2 hours), drying (60° C., relative humidity 40%, 4 hours), and wetting 450° C., relative humidity ≥95%, 2 hours).

In addition, the term “excellent in terms of formability” refers to a case where the elongation after fracture (El) of test pieces in a direction at a right angle to the rolling direction is 28% or more in a tensile test in accordance with JIS Z 2241 and where an average Lankford value (hereinafter, referred to as “average r-value”), which is calculated by using equation (1) below when a test piece is subjected to a strain of 15% in a tensile test in accordance with JIS Z 2241, is 0.70 or more.

Average r-value=(r _(L)+2×r _(D) +r _(C))/4   (1)

Here, r_(L) denotes an r-value determined when a tensile test is performed in a direction parallel to the rolling direction, r_(D) denotes an r-value determined when a tensile test is performed in a direction at an angle of 45° to the rolling direction, and r_(C) denotes an r-value determined when a tensile test is performed in a direction at a right angle to the rolling direction.

Moreover, the term “excellent in terms of ridging resistance” refers to a case where a ridging height which is determined by using the method described below is 2.5 μm or less. First, in order to determine ridging height, a JIS No. 5 tensile test piece is taken in a direction parallel to the rolling direction. Subsequently, after having polished the surface of the taken test piece by using #600 emery paper, the test piece is subjected to a tensile strain of 20%. Subsequently, arithmetic average waviness (Wa) is determined in accordance with JIS B 0601 (2001) in a direction at a right angle to the rolling direction in the polished surface of the central portion of the parallel portion of the test piece by using a surface roughness meter. The determination is performed under the conditions of an observation length of 16 mm, a wavelength of a high-frequency cutoff filter of 0.8 mm, and a wavelength of a low-frequency cutoff filter of 8 mm. The arithmetic average waviness is defined as ridging height.

Solution to Problem

From the results of investigations conducted in order to solve the problems, it was found that it is possible to obtain ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance by performing hot-rolled-sheet annealing in a temperature range in which a dual phase composed of a ferrite phase and an austenite phase is formed after having performed hot rolling and before performing cold rolling on ferritic stainless steel having an appropriate chemical composition, and by further performing cold-rolled-sheet annealing at a higher temperature than before within a temperature range in which a ferrite single phase is formed.

The disclosed embodiments have been completed on the basis of the knowledge described above, and the subject matter of the disclosed embodiments is as follows.

[1] Ferritic stainless steel having a chemical composition containing, by mass %, C: 0.005% or more and 0.035% or less, Si: 0.25% or more and less than 0.40%, Mn: 0.05% or more and 0.35% or less, P: 0.040% or less, S: 0.01% or less, Cr: 15.5% or more and 18.0% or less, Al: 0.001% or more and 10% or less, N: 0.01% or more and 0.06% or less, and the balance being Fe and inevitable impurities, in which Si and Mn satisfy the relationaa expression 29.5×Si−50×Mn+6≥0 (where Si and Mn in the relational expression respectively denote the contents (mass %) of the corresponding chemical elements).

[2] The ferritic stainless steel according to item [1] above, the steel having the chemical composition further containing, by mass %, one, two, or more selected from among Cu: 0.1% or more and 0.5% or less, Ni: 0.1% or more and 0.6% or less, Mo: 0.1% or more and 0.5% or less, and Co: 0.01% or more and 0.3% or less.

[3] The ferritic stainless steel according to item [1] or above, the steel having the chemical composition further containing, by mass n, one, two, or more selected from among V: 0.01% or more and 0.10% or less, Ti: 0.001% or more and 0.05% or less, Nb: 0.001% or more and 0.05% or less, Ca: 0.0002% or more and 0.0020% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, and REM: 0.01% or more and 0.10% or less.

[4] The ferritic stainless steel according to any one of items [1] to [3] above, the steel having an elongation after fracture in a direction at a right angle to the rolling direction of 28% or more, an average Lankford value. of 0.70 or more, and a ridging height of 2.5 μm or less.

[5] A method for manufacturing the ferritic stainless steel according to any one of items [1] to [4] above, the method including performing hot rolling on a steel slab, then performing annealing in which the hot-rolled steel sheet is held at a temperature of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less, then performing cold rolling, and then performing annealing in which the cold-rolled steel sheet is held at a temperature of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.

Here, in the present description, % used when describing the chemical composition of steel shall always refer to mass %.

Advantageous Effects

According to the disclosed embodiments, it is possible to obtain ferritic stainless steel having sufficient corrosion resistance and excellent formability and ridging resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the evaluation results of ductility arranged in accordance with the contents of Si and Mn.

DETAILED DESCRIPTION

Hereafter, the disclosed embodiments will be described in detail.

The ferritic stainless steel according to the disclosed embodiments is subjected to press forming so as to be used in various applications such as architectural parts, parts of home electrical appliances, kitchen appliances, and automobile parts. A material to be used in such applications is required to have sufficient formability (high elongation and a large average r value).

For example, in the case of a spherical-shaped ventilator exit hood, which is formed by bulge forming, if elongation property is insufficient, it is not possible to complete the forming due to the occurrence of necking or fracture in the direction having the lowest elongation during forming process. In addition, since the thickness of a bulging portion after forming varies widely depending on the direction of the steel sheet before forming, there may be a deterioration in the surface appearance of the product. For other example, in the case of a large-size pan, which is manufactured by drawing, if that average r-value is low, it is not possible to obtain a specified product shape due to the occurrence of necking or fracture. There may be a problem regarding heat transfer due to a wide variation in thickness depending on the portion of the pan body. Therefore, high elongation and a large average r-value are required.

Among ferritic stainless steels, for example, SUS430LX (16-mass %-Cr-0.15-mass %-Ti 16-mass %-Cr-0.4-mass %-Nb) and SUS436L (18-mass %-Cr-1.0-mass %-Mo-0.25-mass %-Ti) prescribed in Japanese Industrial Standards JIS G 4305, which contain large amounts of Ti and Nb so as to have excellent formability corresponding to high El and a large average r-value, are used in many applications. However, since these steel grades contain large amounts of Ti and Nb, there is an increase in raw material costs and manufacturing costs, which results in a problem of an increase in price. On the other hand, SUS430 (16-mass %-Cr), which is manufactured in the largest amount among ferritic stainless steels and which does not contain a large amount of Ti or Nb so as to be less expensive than SUS430LX or SUS436L, is poorer in terms of formability than SUS430LX or SUS436L. Therefore, there is a demand for SUS430 having increased formability.

On the other hand, as described above, since surface ruggedness called ridging occurs due to forming work strain the surface of a steel sheet in the case of ferritic stainless steel, a polishing process is necessary in order to remove the surface ruggedness in the case of a product for which aesthetic surface appearance quality is required, which results in a problem of an increase in manufacturing costs. Since a colony, which causes ridging, is more likely to be formed in the case of steel which contains only a small amount of solid solute carbon due to the addition of Ti and Nb, SUS430LX and SUS 436L are poorer than SUS430 in ridging resistance.

As described above, it is a fact that a technique for manufacturing ferritic stainless steel which has sufficient corrosion resistance, excellent formability, and excellent ridging resistance at the same time and which is inexpensive has not yet been well established.

Therefore, the present inventors diligently conducted investigations regarding a method for manufacturing ferritic stainless steel which has an appropriate chemical composition (in particular, SUS430-based (16-mass %-Cr-based) chemical composition) without containing a large amount of Ti or Nb and which satisfies the relational expressions El≥28%, average r-value≥0.70, and ridging height≥2.5 μm. In addition, although there are a box annealing method (batch annealing method) and a continuous annealing method a method for performing annealing (hereinafter, referred to as “hot-rolled-sheet annealing”) on a ferritic stainless steel sheet after a hot rolling process and before a cold rolling process, consideration was given to achieving the specified formability by using a continuous annealing method, which has a high productivity, instead of a box annealing method, which has a low productivity as a result of taking a long time.

There is a problem with a conventional technique using a continuous annealing furnace in that, since annealing is performed in a temperature range in which a metallographic structure consisting of a ferrite single phase is formed, insufficient recrystallization occurs, which results in insufficient elongation and in poor ridging resistance due to a colony being retained after a finish annealing process. Therefore, the present inventors devised performing hot-rolled-sheet annealing in a dual-phase temperature range in which a ferrite phase and an austenite phase are formed, then performing cold rolling by using an ordinary method, and then performing finish annealing (cold-rolled-sheet annealing) at a higher temperature than before in order to finally form a ferrite single phase metallographic structure again. Specifically, the method is as follows. By performing hot-rolled-sheet annealing in a dual-phase temperature range in which a ferrite phase and an austenite phase are formed and which is higher than a temperature range in which a ferrite single phase structure is formed, an austenite phase, which is formed from a ferrite phase in the hot-rolled-sheet annealing process, is formed so as to have a crystal orientation different from that of a ferrite phase before the annealing process. In addition, since a metallographic structure formed after the hot-rolled-sheet annealing process is composed of a ferrite phase and a martensite phase which is formed through the transformation of an austenite phase in a cooling process, the strain caused by cold-rolling tends to be concentrated at the interface between the different phases, that is, a soft ferrite phase and a hard martensite phase, which results in the formation of recrystallization sites where recrystallization occurs in a cold-rolled-sheet annealing process. As a result, since the colony of a ferrite phase is effectively broken, there is an increase in ridging resistance. By performing cold-rolling, and by further performing cold-rolled-sheet annealing in a temperature range in which a ferrite single phase metallographic structure is formed in order to decompose a martensite phase into carbonitrides and a ferrite phase, it is possible to achieve excellent ridging resistance corresponding to a ridging height of 2.5 μm or less.

However, it was found that it is not possible to stably achieve excellent formability with only the technique described above. Therefore, close investigations regarding the influence of various constituent chemical elements and manufacturing conditions on formability were conducted. As a result, it was found that, by controlling the chemical composition of steel and a cold-rolled-sheet annealing temperature to be within preferable ranges, it is possible to stably achieve excellent formability. That is, by controlling the contents of Si, which is a ferrite-forming element, and Mn, which is an austenite-forming element, to be within preferable ranges, the lower limit (hereinafter, also referred to as “the T_(A) point”) of a temperature range in which an austenite phase is formed, is raised. With this, cold-rolled-sheet annealing is performed at a higher temperature in order to further promote grain growth. As a result, it was found that, since a ferrite single phase metallographic structure including sufficiently grown grains is formed in a metallographic structure after a cold-rolled-sheet annealing process, it is possible to achieve excellent formability corresponding to an elongation after fracture (El) of 28% or more and an average r-value of 0.70 or more and excellent ridging resistance at the same time.

Hereafter, the chemical composition of the ferritic stainless steel according to the disclosed embodiments will be described. Hereinafter, % shall refer to mass %, unless otherwise noted.

C: 0.005% or more and 0.035% or less

C is effective for expanding a dual-phase temperature range in which a ferrite phase and an austenite phase are formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. In order to realize such an effect, it is necessary that the C content be 0.005% or more. However, in the case where the C content is more than 0.035%, since there is an increase in the hardness of a steel sheet, there is a decrease in ductility. Therefore, the C content is set to be 0.005% or more and 0.035% or less, preferably 0.010% or more and 0.030% or less, or more preferably 0.015% or more and 0.025% or less.

Si: 0.25% or more and less than 0.40%

Si is an element which raises the T_(A) point. In order to realize such an effect, i.t is necessary that the Si content be 0.25% or more. However, in the case where the Si content is 0.40% or more, since there is an increase in the hardness of a steel sheet, there is an increase it rolling load in a hot rolling process, and there is a decrease in ductility after a cold-rolled-sheet annealing process, which makes it impossible to achieve the specified elongation after fracture. Therefore, the Si content is set to be 0.25% or more and less than 0.40%, preferably 0.25% or more and 0.35% or less, or more preferably 0.25% or more and 0.30% or less.

Mn: 0.05% or more and 0.35% or less

Mn is, like C, effective for expanding a dual-phase temperature range in which a ferrite phase and an austenite phase are formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. In order to realize such an effect, it is necessary that the Mn content be 0.05% or more. However, in the case where the Mn content is more than 0.35%, since there is an excessive fall in the T_(A) point, it is not possible to achieve the specified elongation after fracture. Therefore, the Mn content is set to be 0.05% or more and 0.35% or less, preferably 0.10% or more and 0.30% or less, or more preferably 0.15% or more and 0.25% or less

29.5×Si−50×Mn+6≥0 (where Si and Mn in the relational expression respectively denote the contents (mass %) of the corresponding elements)

In the disclosed embodiments, controlling the lower limit (the T_(A) point) of a temperature range in which an austenite phase is formed is a very important factor. As described above, the disclosed embodiments are greatly characterized in that hot-rolled-sheet annealing performed in a dual-phase temperature range in which ferrite phase and an austenite phase are formed in order to form a metallographic structure including a dual phase composed of a ferrite phase and a martensite phase after a hot-rolled-sheet annealing process, and in that cold-rolled-sheet annealing following a cold rolling process is performed in a temperature range in which a ferrite single phase is formed in order to finally form a ferrite single phase metallographic structure. In order to achieve excellent elongation after fracture by using this method, it is necessary that a martensite phase, which has been formed by performing hot-rolled-sheet annealing, be removed by decomposition into a ferrite phase and precipitates by performing cold-rolled-sheet annealing it order to form a metallographic structure including a ferrite single phase and that ferrite grain diameter be increased through sufficient grain growth. From the results of investigations conducted by the present inventors, it was found that the decomposition of a martensite phase, which has been formed by performing hot-rolled-sheet annealing, rapidly progresses in a temperature range of about 750° C. or higher in a cold-rolled-sheet annealing process following a cold rolling process and that ferrite grains are formed through recrystallization in a temperature range of 800° C. or higher.

It is widely known that, the higher the temperature or the longer the annealing time, the more the crystal grains grow. However, since cold-rolled-sheet annealing is performed by using a continuous annealing furnace, there is a significant decrease in productivity in the case where annealing time is long. On the other hand, performing cold-rolled-sheet annealing at a high temperature on steel having a chemical composition according to conventional techniques means performing annealing in a dual-phase temperature range in which a ferrite phase and an austenite phase are formed. In this case, since an austenite phase is newly formed in the metallographic structure, this austenite phase transforms into a martensite phase after a cooling process, which makes it impossible to achieve the specified elongation after fracture due to an increase in the hardness of a steel sheet.

Therefore, in the disclosed embodiments, by controlling the chemical composition of steel so that the lower limit (the T_(A) point) of a temperature range in which austenite is formed is higher than before, it is possible to perform annealing at a higher temperature in a temperature range in which a ferrite single phase is formed. With this, it is possible to sufficiently grow ferrite grains without forming an austenite phase.

Specifically, the T_(A) point is raised by controlling the contents of Si and Mn to be within preferable ranges. Since Si is a ferrite-forming element, an increase in the Si content raises the T_(A) point. On the other hand, Mn is an austenite-forming element, an increase in the Mn content lowers the T_(A) point. From the results of investigations, it was found that, in the case where (29.5×Si−50×Mn+6) is less than 0, since there is an insufficient rise in T_(A) point, it is not possible to achieve the specified elongation after fracture. Therefore, in the disclosed embodiments, (29.5×Si−50×Mn 6) is set to be 0 or more.

P: 0.040% or less

Since P is an element which promotes intergranular fracturing due to intergranular segregation, it is preferable that the P content be as small as possible, and the upper limit of the P content is set to be 0.040%, preferably 0.035% or less, or more preferably 0.030% or less.

S: 0.01% or less

S is an element which decreases, for example, ductility and corrosion resistance as a result of existing in the form of sulfide-based inclusions such as MnS, and such a negative effect becomes marked, in particular, in the case where the S content is more than 0.01%. Therefore, it is preferable that the S content be as small as possible, and the upper limit of the S content is set to be 0.01% or less, preferably 0.007% or less, or more preferably 0.005% or less, in the disclosed embodiments.

Cr: 15.5% or more and 18.0% or less

Cr is an element which is effective for increasing corrosion resistance by forming a passivation film on the surface of a steel sheet. In order to realize such an effect, it is necessary that the Cr content be 15.5% or more. However, in the case where the Cr content is more than 18.0%, since there is an insufficient amount of austenite phase formed in a hot-rolled-sheet annealing process, it is not possible to achieve the specified ridging resistance. Therefore, the Cr content is set to be 15.5% or more and 18.0% or less, preferably 16.0% or more and 17.5% or less, or more preferably 16.0% or more and 17.0% or less.

Al: 0.001% or more and 0.10% or less

Al is, like Si, an element which functions as a deoxidizing agent, In order to realize such an effect, it is necessary that the Al content be 0.001% or more, However, in the case where the Al content more than 0.10%, since there is an increase in the amount of Al-based inclusions such as Al₂O₃, there is a tendency for surface quality to decrease. Therefore, the Al content is set to be 0.001% or more and 0.10% or less, preferably 0.001% or more and 0.05% or less, or more preferably 0.001% or more and 0.03% or less.

N: 0.01% or more and 0.06% or less

N is, like C and Mn, effective for expanding a dual-phase temperature range in which a ferrite phase and an austenite phase are formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. In order to realize such an effect, it is necessary that the N content 0.01% or more. However, in the case where the N content is more than 0.06%, there is a significant decrease in ductility, and there is a decrease in corrosion resistance by promoting the precipitation of Cr nitrides. Therefore, the N content is set to be 0.01% or more and 0.06 or less, preferably 0.01% or more and 0.05% or less, or more preferably 0.02% or more and 0.04% or less.

The remainder is Fe and inevitable impurities

Although the effect of the disclosed embodiments is realized by providing the chemical composition described above, the following elements may further be added in order to improve manufacturability and material properties.

One, two, or more selected from among Cu: 0.1% or more and 0.5% or less, Ni: 0.1% or more and 0.6% or less, Mo: 0.1% or more and 0.5% or less, and Co: 0.01% or more and 0.3% or less

Since Cu and Ni are both elements which increase corrosion resistance, adding these chemical elements is effective, in particular, in the case where high corrosion resistance is required. In addition, Cu and Ni are effective for expanding a dual-phase temperature range in which a ferrite phase and an austenite phase are formed in a hot-rolled-sheet annealing process by promoting the formation of an austenite phase. Such effects become marked in the case where the content of each of these elements is 0.1% or more. However, it is not preferable that the Cu content be more than 0.5%, because there may be a decrease in formability. Therefore, in the case where Cu is added, the Cu content is set to be 0.1% or more and 0.5% or less, or preferably 0.2% or more and 0.3% or less. It is not preferable that the Ni content be more than 0.6%, because there is a decrease in formability. Therefore, in the case where Ni is added, the Ni content is set to be 0.1% ox more and 0.6% or less, or preferably 0.1% or more and 0.3% or less.

Since Mo is an element which increases corrosion resistance, adding Mo is effective, in particular, in the case where high corrosion resistance is required. Such an effect becomes marked in the case where the Mo content is 0.1% or more. However, it is not preferable that the Mo content be more than 0.5%, because there is an insufficient amount of austenite phase formed in a hot-rolled-sheet annealing process, which makes it impossible to achieve the specified material properties. Therefore, in the case where Mo is added, the Mo content is set to be 0.1% or more and 0.5% or less, or preferably 0.2% or more and 0.3% or less

Co is an element which increases toughness. Such an effect is realized in the case where the Co content is 0.01% or more. On the other hand, in the case where the Co content is more than 0.3%, there is a decrease in formability. Therefore, in the case where Co is added, the Co content is set to be 0.01% or more and 0.3% or less.

One, two, or more selected from among V: 0.01% or more and 0.10% or less, Ti: 0.001% or more and 0.05% or less, Nb: 0.001% or more and 0.05% or less, Ca: 0.0002% or more and 0.0020% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, and REM: 0.01% or more and 0.10% or less

V: 0.01% or more and 0.10% or less

V decreases the amounts of a solid solute C and a solid solute N as a result of combining with C and N in steel. With this, there is an increase in average r value. Moreover, there is an increase in surface quality by inhibiting the occurrence of linear flaws caused by hot rolling and annealing as a result of controlling the precipitation behavior of carbonitrides in a hot-rolled steel sheet. In order to realize such effects, it is necessary that the V content be 0.01% or more. However, in the case where the V content is more than 0.10%, there is a decrease in workability, and there is an increase in manufacturing costs. Therefore, in the case where V is added, the V content is set to be 0.01% or more and 0.10% or less, or preferably 0.03% or more and 0.08% or less.

Ti: 0.001% or more and 0.05% or less and Nb: 0.001% or more and 0.05% or less

Ti and Nb, which are, like V, elements having a high affinity for C and N, are effective for increasing workability after a cold-rolled-sheet annealing process (finish ling process) by decreasing the amounts of a solid solute C and a solid solute N in a parent phase as a result of being precipitated in the form of carbides or nitrides in a hot rolling process. In order to realize such an effect, it is necessary that the Ti content be 0.001% or more or that the Nb content be 0.001% or more. However, in the case where the content of Ti or Nb is more than 0.05%, it is not possible to achieve good surface quality due to an excessive amount of TiN or NbC precipitated. Therefore, in the case where Ti is added, the Ti content is set to be 0.001% or more and 0.05% or less, or preferably 0.003% or more and 0.020% or less, and, in the case where Nb is added, the Nb content is set to be 0.001% or more and 0.05% or less p eferably 0.005% or more and 0.020% or less, or more preferably 0.010% or more and 0.015% or less.

Ca: 0.0002% or more and 0.0020% or less

Ca is an element which is effective for preventing nozzle clogging which caused by the crystallization of Ti-based inclusions which tend to be formed in a continuous casting process. In order to realize such an effect, it is necessary that the Ca content be 0.0002% or more. However, in the case where the Ca content is more than 0.0020%, there is a decrease in corrosion resistance due to the formation of CaS. Therefore, in the case where Ca is added, the Ca content is set to be 0.0002% or more and 0.0020% or less, preferably 0.0005% or more and 0.0015% or less, or more preferably 0.0005% or more and 0.0010% or less.

Mg: 0.0002% or more and 0.0050% or less

Mg is an element which is effective for increasing hot workability. In order to realize such an effect, it is necessary that the Mg content be 0.0002% or more. However, in the case where the Mg content is more than 0.0050%, there is a decrease in surface quality. Therefore, in the case where Mg is added, the Mg content is set to be 0.0002% or more and 0.0050% or less, preferably 0.0005% or more and 0.0035% or less, or more preferably, 0.0005% or more and 0.0020% or less.

B: 0.0002% or more and 0.0050% or less

B is an element which is effective for preventing secondary cold work embrittlement. in order to realize such an effect, it is necessary that the B content be 0.0002% or more. However, in the case where the B content is more than 0.0050%, there is a decrease in hot workability. Therefore, in the case where B is added, the B content is set to be 0.0002% or more and 0.0050% or less, preferably 0.0005% or more and 0.0035% or less, or more preferably 0.0005% or more and 0.0020% or less.

REM: 0.01% or more and 0.10% or less

REM is an element which increases oxidation resistance and which is effective for, in particular, increasing the corrosion resistance of a weld zone by inhibiting the formation of an oxide film in the weld zone. In order to realize such an effect, it is necessary that the REM content be 0.01% or more. However, in the case where the REM content is more than 0.10%, there is a decrease in manufacturability such as pickling capability in a cold-rolled-sheet annealing process. In addition, since REM is an expensive element, it is not preferable that an excessively large amount of REM be added, because there is an increase in manufacturing costs. Therefore, in the case where REM is added, the REM content is set to be 01% or more and 0.10% or less.

Hereafter, the method for manufacturing the ferritic stainless steel according to the disclosed embodiments will be described.

It is possible to obtain the ferritic stainless steel according to the disclosed embodiments by performing hot rolling on a steel slab having the chemical composition described above, by then performing annealing in which the hot-rolled steel sheet is held at a temperature of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less, by then performing cold rolling, and by then performing annealing in which the cold-rolled steel sheet is held at a temperature of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.

First, after having prepared molten steel by using a known method such as one using a converter, an electric furnace, or a vacuum melting furnace, a steel material (slab) is obtained by using a continuous casting method or an ingot casting-slabbing method. This slab is made into a hot-rolled steel sheet by performing hot rolling after having heated the slab at a temperature of 1100° C. or higher and 1250° C. or lower for one hour or more and 24 hours or less or by performing hot rolling directly on the slab in the cast state without heating the slab.

Subsequently, hot-rolled-sheet annealing is performed in a temperature range of 900° C. or higher and 1050° C. or lower, that a dual-phase temperature range in which a ferrite phase and an austenite phase are formed for 5 seconds or more and 15 minutes or less.

Subsequently, after having performed pickling as needed, cold rolling and cold-rolled-sheet annealing are performed. A product is obtained by further performing pickling as needed.

It is preferable that cold rolling be performed with a rolling reduction of 50% or more from the viewpoint of elongation property, bendability, press formability, and shape correction. In addition, in the disclosed embodiments, cold rolling-annealing may be repeated twice or more.

Here, for example, grinding or polishing may be further performed in order to increase surface quality.

The reasons for the limitations on the manufacturing conditions will be described hereafter.

Hot-rolled-sheet annealing in which a hot-rolled steel sheet is held at a temperature of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less

A hot-rolled-sheet annealing process is a very important process for achieving excellent formability and ridging resistance in the disclosed embodiments. In the case where the hot-rolled-sheet annealing temperature is lower than 900° C., since recrystallization does not occur sufficiently, and since annealing is performed in a temperature range in which a ferrite single phase is formed, there is a case where it is not possible to obtain the effect of the disclosed embodiments which is provided by performing annealing in a dual-phase temperature range. However, in the case where the hot-rolled-sheet annealing temperature is higher than 1050° C., since the formation of the solid solution of carbides is promoted, concentration of C into an austenite phase is promoted, which may result in a decrease in surface quality due to the formation of a hard martensite phase after a hot-rolled-sheet annealing process. In the case where the annealing time is less than 5 seconds, since the formation of an austenite phase or the recrystallization of a ferrite phase does not occur sufficiently even if annealing is performed at the specified temperature, there is a case where it is not possible to achieve the desired formability. On the other hand, in the case where the annealing time is more than 15 minutes, concentration of C into an austenite phase is promoted, which may result in a decrease in surface quality by the same mechanism as that described above. Therefore, hot-rolled-sheet annealing should be performed at a temperature of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less, preferably at a temperature of 920° C. or higher and 1020° C. or lower for 15 seconds or more and 5 minutes or less, or more preferably at a temperature of 920° C. or higher and 1000° C. or lower for 30 seconds or more and 3 minutes or less.

Cold-rolled-sheet annealing in which a cold-rolled steel sheet is held at a temperature of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less

A cold-rolled-sheet annealing process is an important process for transforming a dual phase structure composed of a ferrite phase and a martensite phase, which has been formed in a hot-rolled-sheet annealing process, into a ferrite single phase structure. In the case where the cold-rolled-sheet annealing temperature is lower than 800° C., since recrystallization does not occur sufficiently, it is not possible to achieve the specified elongation after fracture or average r-value. On the other hand, in the case where the cold-rolled-sheet annealing temperature is higher than 950° C., if this temperature is in a dual-phase temperature range for the treated steel component in which a ferrite phase and an austenite phase are formed, since a martensite phase formed after a cold-rolled-sheet annealing process, it is not possible to achieve the specified elongation after fracture due to an increase in the hardness of a steel sheet. In addition, even if this temperature is in a temperature range in which a ferrite single phase is formed for the treated steel component, it is not preferable that cold-rolled-sheet annealing be performed at this temperature from the viewpoint of surface quality, because there is a decrease in the glossiness steel sheet due to a significant increase in crystal grain diameter. In the case where the annealing time is less than seconds, even if annealing is performed at the specified temperature, since the recrystallization of a ferrite phase does not occur sufficiently, it is not possible to achieve the specified elongation after fracture or average r-value. It is not preferable that the annealing time be more than 5 minutes from the viewpoint of surface quality, because there is a decrease in the glossiness of a steel sheet due to a significant increase in crystal grain diameter. Therefore, cold-rolled-sheet annealing should be performed at a temperature of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less, or preferably at a temperature of 850° C. or higher and 900° C. or lower for 15 seconds or more and 3 minutes or less. BA annealing (bright annealing) may be performed in order to achieve a higher level of glossiness.

EXAMPLE 1

Hereafter, the disclosed embodiments will be described in detail on the basis of examples.

Molten stainless steels having the chemical compositions given in Table 1 were prepared by using a small-size vacuum melting furnace having a capacity of 50 kg. These steel ingots were heated at a temperature of 1150° C. for one hour and then subjected to hot rolling in order to obtain hot-rolled steel sheets having a thickness of 3.5 mm. Subsequently these hot-rolled steel sheets were subjected to hot-rolled-sheet annealing under the conditions given in Table 2.

Subsequently, descaling involving a shot blasting treatment and pickling was performed on the surface. Moreover, by performing cold rolling in order to obtain a thickness of 0.7 mm, then by performing cold-rolled-sheet annealing (finish annealing) under the conditions given in Table 2, and by then performing a descaling treatment involving pickling, cold-rolled, pickled, and annealed steel sheets were obtained.

The cold-rolled, pickled, and annealed steel sheets obtained as described above were evaluated as follows.

(1) Evaluation of Ductility

By performing a tensile test in accordance with JIS Z 2241 on a JIS No. 13B tensile test piece taken from the cold-rolled, pickled, and annealed steel sheet in a direction at a right angle to the rolling direction, and by determining elongation after fracture, a case where the elongation after fracture in each direction was 28% or more was judged as satisfactory (◯), and a case where the elongation after fracture in any one direction was lower than 28% was judged as unsatisfactory (×).

(2) Evaluation of Average R-Value

By performing a tensile test in accordance with JIS Z 2411 on JIS No. 13B tensile test pieces taken from the cold-rolled, pickled, and annealed steel sheet respectively in a direction (L-direction) parallel to the rolling direction, a direction (D-direction) at 45° to the rolling direction, and a direction (C-direct ion) at a right angle to the rolling direction until a strain of 15% being applied, and by determining the r-value of each of the three directions, an average r-value (=(r_(L)+2r_(D)+r_(c))/4) was calculated. Here, r_(L), r_(D), and r_(c) respectively denote the r-values in the L-direction, the D-direction, and the C-direction. A case of an average r-value of 0.70 or more was judged as satisfactory (◯), and a case of an average r value of less than 0.70 was judged as unsatisfactory (×).

(3) Evaluation of Ridging Resistance

After having applied a tensile strain of 20% to a JIS NO. 5 tensile test piece which had been taken from the cold-rolled, pickled, and annealed steel sheet in a direction parallel to the rolling direction and whose surface had been polished by using #600 emery paper, arithmetic average waviness (Wa) prescribed in JIS B 0601 (2001) in a direction at a right angle to the rolling direction was determined in the polished surface of the central portion of the parallel part of the test piece by using a surface roughness meter with a determination length of 16 mm, a wavelength of a high-frequency cutoff filter of 0.8 mm, a wavelength of a low-frequency cutoff filter of 8 mm. A case where the arithmetic average waviness (Wa) was 2.5 μm or less was judged as satisfactory (◯), and a case where the arithmetic average waviness (Wa) was more than 2.5 μm was judged as unsatisfactory (×).

(4) Evaluation of Corrosion Resistance

A salt spray cyclic corrosion test prescribed in JIS H 8502 was performed on a test piece of 60 mm×100 mm which had been taken from the cold-rolled, pickled, and annealed steel sheet, whose surface had been polished by using #600 emery paper, and whose end surfaces were sealed. The salt spray cyclic corrosion test was performed 8 cycles, where a unit cycle consists of salt spraying (with 5-mass %-NaCl, at 35° C., and for 2 hours), drying (at 60° C., for 4 hours, and under a relative humidity of 40%), and wetting (at 50° C., for 2 hours, and under a relative humidity of 95% or more).

By taking the photograph of the surface of the test piece after the salt spray cyclic corrosion test had been performed 8 cycles, by determining the rust area on one side of the test piece by performing image analysis, a rust area ratio ((rust area in the test piece)/(total area of the test piece)×100 [%]) was calculated as the ratio of the rust area to the total area of the test piece. A case where the rust area ratio was 10% or less was judged as a satisfactory case of excellent corrosion resistance (⊙), a case where the rust area ratio was more than 10% and 25% or less was judged as satisfactory (◯), and a case where the rust area ratio was more than 25% was judged as unsatisfactory (×).

The evaluation results are given in Table 2 along with the annealing conditions. In addition, a graph illustrating the evaluation results of ductility of Nos. 1 through 25, whose Cr contents were within the range according to the disclosed embodiments, arranged in accordance with the contents of Si and Mn is given in FIG. 1.

TABLE 1 mass % Steel 29.5 × Si − Code C Si Mn P S Cr Al N 50 × Mn + 6 Other Note AA 0.019 0.39 0.29 0.022 0.004 16.4 0.003 0.03 3 Example AB 0.020 0.37 0.09 0.028 0.006 16.2 0.004 0.03 12 Example AC 0.018 0.39 0.34 0.026 0.005 16.5 0.002 0.04 1 Ni: 0.4 Example AD 0.035 0.26 0.08 0.031 0.005 17.7 0.003 0.05 10 Example AE 0.022 0.29 0.22 0.035 0.006 16.2 0.003 0.04 4 Ni: 0.1 Example AF 0.021 0.34 0.24 0.032 0.005 16.4 0.004 0.04 4 Cu: 0.4 Example AG 0.022 0.37 0.32 0.023 0.006 16.3 0.005 0.05 1 Mo: 0.4 Example AH 0.018 0.34 0.29 0.038 0.003 16.7 0.003 0.03 2 Co: 0.3 Example AI 0.023 0.33 0.28 0.033 0.003 16.5 0.005 0.03 2 V: 0.09 Example AJ 0.020 0.32 0.20 0.029 0.005 16.4 0.003 0.03 5 Ni: 0.1 Example Nb: 0.04 AK 0.024 0.31 0.28 0.028 0.003 16.1 0.003 0.02 1 Ti: 0.03 Example B: 0.0023 AL 0.021 0.33 0.27 0.034 0.002 16.3 0.004 0.03 2 REM: 0.04 Example AM 0.025 0.30 0.26 0.021 0.003 16.2 0.005 0.03 2 Mg: 0.0017 Example AN 0.009 0.33 0.15 0.026 0.004 15.7 0.003 0.04 8 Example AO 0.012 0.27 0.17 0.039 0.002 16.1 0.003 0.04 5 Ni: 0.1 Example AP 0.009 0.25 0.24 0.038 0.003 15.7 0.004 0.03 1 Ni: 0.2 Example AQ 0.008 0.39 0.21 0.039 0.003 16.4 0.003 0.4 7 Example BA 0.017 0.21 0.19 0.031 0.005 16.6 0.004 0.04 3 Comparative Example BB 0.026 0.42 0.32 0.034 0.003 16.1 0.003 0.04 2 Comparative Example BC 0.024 0.26 0.29 0.034 0.006 16.2 0.003 0.04 −1 Ni: 0.1 Comparative Example BD 0.022 0.29 0.32 0.030 0.004 16.7 0.003 0.04 −1 Comparative Example BE 0.025 0.31 0.32 0.027 0.004 16.0 0.005 0.04 −1 Comparative Example BF 0.021 0.34 0.34 0.028 0.004 16.2 0.005 0.03 −1 B: 0.0018 Comparative Example BG 0.025 0.38 0.37 0.033 0.005 16.0 0.004 0.04 −1 Comparative Example BH 0.022 0.23 0.34 0.031 0.004 16.7 0.003 0.04 −4 Comparative Example BI 0.021 0.38 0.27 0.024 0.005 15.4 0.004 0.03 4 Comparative Example BJ 0.023 0.26 0.27 0.030 0.007 18.7 0.004 0.05 0 Comparative Example An underlined portion indicates a value out of the range according to the disclosed embodiments.

TABLE 2 elong- Corro- ation Average Ridging sion Steel Hot-rolled sheet Cold-rolled sheet after r Resist- Resist- No. Code Annealing Condition Annealing Condition fracture Value ance ance Note 1 AA Holding at 940° C. Holding at 860° C. ○ ○ ○ ○ Example for 60 seconds for 30 seconds 2 AB Ditto Ditto ○ ○ ○ ○ Example 3 AC Ditto Ditto ○ ○ ○ ⊙ Example 4 AD Ditto Ditto ○ ○ ○ ⊙ Example 5 AE Ditto Ditto ○ ○ ○ ○ Example 6 AF Ditto Ditto ○ ○ ○ ⊙ Example 7 AG Ditto Ditto ○ ○ ○ ⊙ Example 8 AH Ditto Ditto ○ ○ ○ ○ Example 9 AI Ditto Ditto ○ ○ ○ ○ Example 10 AJ Ditto Ditto ○ ○ ○ ○ Example 11 AK Ditto Ditto ○ ○ ○ ○ Example 12 AL Ditto Ditto ○ ○ ○ ○ Example 13 AM Ditto Ditto ○ ○ ○ ○ Example 14 AN Ditto Ditto ○ ○ ○ ○ Example 15 AO Ditto Ditto ○ ○ ○ ○ Example 16 AP Difto Ditto ○ ○ ○ ○ Example 17 AQ Ditto Ditto ○ ○ ○ ○ Example 18 BA Ditto Ditto x ○ ○ ○ Comparative Example 19 BB Ditto Ditto x ○ ○ ○ Comparative Example 20 BC Ditto Ditto x ○ ○ ○ Comparative Example 21 BD Ditto Ditto x ○ ○ ○ Comparative Example 22 BE Ditto Ditto x ○ ○ ○ Comparative Example 23 BF Ditto Ditto x ○ ○ ○ Comparative Example 24 BG Ditto Ditto x ○ ○ ○ Comparative Example 25 BH Ditto Ditto x ○ ○ ○ Comparative Example 26 BI Ditto Ditto ○ ○ ○ x Comparative Example 27 BJ Ditto Ditto ○ ○ x ⊙ Comparative Example 28 BI Holding at 880° C. Holding at 860° C. ○ ○ x x Comparative for 60 Seconds for 30 Seconds Example 29 BI Holding at 940° C. Holding at 780° C. x x ○ x Comparative for 60 Seconds for 30 Seconds Example 30 BI Holding at 940° C. Holding at 970° C. x ○ ○ x Comparative for 60 Seconds for 30 Seconds Example

It is clarified that Nos. 1 through 17 (steels AA through AQ), whose chemical compositions were within the range according to the disclosed embodiments, had excellent formability and ridging resistance corresponding to an elongation after fracture of 28% or more, an average r-value of 0.70 or more, and a ridging height of 2.5 μm or less. Moreover, any one of Nos. 1 through 17 had good corrosion resistance corresponding to a rust area ratio on one side of a test piece of 25% or less after the salt spray cyclic corrosion test had been performed 8 cycles.

In particular, No. 3 (steel AC) containing 0.4% of Ni, No. 4 (steel AD) containing 17.7% of Cr, No. 6 (steel AF) containing 0.4% of Cu, and No. 7 (steel AG) containing 0.4% of Mo had a much higher level of corrosion resistance corresponding to a rust area ratio of 10% or less after the salt spray cyclic corrosion test had been performed.

On the other hand, in the case of No. 26 (steel BI), where the Cr content was less than the range according to the disclosed embodiments, although the specified formability and ridging resistance were achieved, it was not possible to achieve the specified corrosion resistance due to insufficient Cr content.

In the case of No. 27 (steel BJ) where the Cr content was more than the range according to the disclosed embodiments, although sufficient corrosion resistance was achieved, since an austenite phase was not formed in the hot-rolled-sheet annealing process due to excessive Cr content, it was not possible to achieve the specified ridging resistance.

In the case of Nos. 18 (steel BA) and 25 (steel BH) where the Si content was less than the range according to the disclosed embodiments, since an austenite phase was formed in the cold-rolled-sheet annealing process due to insufficient Si content, there was an increase in the hardness of the steel sheet as a result of this austenite phase transforming into a martensite phase after a cooling process, which made it impossible to achieve the specified elongation after fracture.

In the case of No. 19 (steel BB) where the Si content was more than the range according to the disclosed embodiments, since there is an increase in the hardness of the steel sheet due to excessive Si content, it was not possible to achieve the specified elongation after fracture.

In the case of Nos. 20 through 23 (steels BC through BF) where (29.5×Si−50×Mn+6) was less than the range according to the disclosed embodiments while the contents of Si and Mn are within the ranges according to the disclosed embodiments, since an austenite phase was formed in the cold-rolled-sheet annealing process due to the inappropriate balance between the contents of Si and Mn, there was an increase in the hardness of the steel sheet as a result of this austenite phase transforming into a martensite phase after a cooling process, which made it impossible to achieve the specified elongation after fracture.

In the case of No. 24 (steel BG) where the Mn content was more than the range according to the disclosed embodiments, since an austenite phase was formed in the cold-rolled-sheet annealing process as a result of (29.5×Si−50×Mn+6) being less than the range according to the disclosed embodiments due to excessive Mn content, there was an increase in the hardness of the steel sheet as a result of this austenite phase transforming into a martensite phase after a cooling process, which made it impossible to achieve the specified elongation after fracture. Among the evaluation results describe above, a graph illustrating the evaluation results of ductility of Nos. 1 through 25, whose Cr contents were within the range according to the disclosed embodiments, arranged in accordance with the contents of Si and Mn is given in FIG. 1. It is clarified that it is possible to achieve the specified elongation after fracture in the case where not only the contents of Si and Mn but also (29.5×Si−50×Mn+6) are within the ranges according to the disclosed embodiments.

In Nos. 28 through 30, by using steel BI, with which it was not possible to achieve the specified corrosion resistance due to insufficient Cr content while the specified formability and ridging resistance were achieved, the influences of a hot-rolled-sheet annealing temperature and a cold-rolled-sheet annealing temperature on formability and ridging resistance were investigated. In the case of No. 28 where the hot-rolled-sheet annealing temperature was 880° C., which was lower than the range according to the disclosed embodiments, since an austenite phase was not formed in the hot-rolled-sheet annealing process, it was not possible to achieve the specified ridging resistance. In the case of No. 29 where the cold-rolled-sheet annealing temperature was 780° C., which was lower than the range according to the disclosed embodiments, since the grain growth of ferrite grains in the cold-rolled-sheet annealing process did not progress sufficiently, it was not possible to achieve the specified formability (elongation after fracture and average r-value). In the case of No. 30 where the cold-rolled-sheet annealing temperature was 970° C., which was higher than the range according to the disclosed embodiments, since an austenite phase was formed in the cold-rolled-sheet annealing process, there was an increase in the hardness of the steel sheet as a result of this austenite phase transforming into a martensite phase after a cooling process, which made it impossible to achieve the specified elongation after fracture.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel obtained by using the disclosed embodiments can particularly preferably be used for products manufactured by performing press forming involving mainly drawing and in applications in which high aesthetic surface appearance quality is required such as kitchen appliances and tableware. 

1. Ferritic stainless steel having a chemical composition comprising, by mass %: C: 0.005% or more and 0.035% or less; Si: 0.25% or more and less than 0.40%; Mn: 0.05% or more and 0.35% or less; P: 0.040% or less; S: 0.01% or less; Cr: 15.5% or more and 18.0% or less; Al: 0.001% or more and 0.10% or less; N: 0.01% or more and 0.06% or less; and the balance being Fe and inevitable impurities, wherein Si and Mn satisfy the relational expression (1) 29.5×Si−50×Mn+6≥0   (1) (where Si and Mn respectively denote the content, by mass %, of the corresponding chemical element.
 2. The ferritic stainless steel according to claim 1, the chemical composition further comprising, by mass %, at least one selected from the group consisting of Cu: 0.1% or more and 0.5% or less, Ni: 0.1% or more and 0.6% or less, Mo: 0.1% or more and 0.5% or less, and Co: 0.01% or more and 0.3% or less.
 3. The ferritic stainless steel according to claim 1, the chemical composition further comprising, by mass %, at least one selected from the group consisting of V: 0.01% or more and 0.10% or less, Ti: 0.001% or more and 0.05% or less, Nb: 0.001% or more and 0.05% or less, Ca: 0.0002% or more and 0.0020% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, and REM: 0.01% or more and 0.10% or less.
 4. The ferritic stainless steel according to claim 1, wherein the steel has an elongation after fracture in a direction at a right angle to a rolling direction of 28% or more, an average Lankford value of 0.70 or more, and a ridging height of 2.5 μm or less.
 5. A method for manufacturing the ferritic stainless steel according to claim 1, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 6. The ferritic stainless steel according to claim 2, the chemical composition further comprising, by mass %, at least one selected from the group consisting of V: 0.01% or more and 0.10% or less, Ti: 0.001% or more and 0.05% or less, Nb: 0.001% or more and 0.05% or less, Ca: 0.0002% or more and 0.0020% or less, Mg: 0.0002% or more and 0.0050% or less, B: 0.0002% or more and 0.0050% or less, and REM: 0.01% or more and 0.10% or less.
 7. The ferritic stainless steel according to claim 2, wherein the steel has an elongation after fracture in a direction at a right angle to a rolling direction of 28% or more, an average Lankford value of 0.70 or more, and a ridging height of 2.5 μm or less.
 8. The ferritic stainless steel according to claim 3, wherein the steel has an elongation after fracture in a direction at a right angle to a rolling direction of 28% or more, an average Lankford value of 0.70 or more, and a ridging height of 2.5 μm or less.
 9. The ferritic stainless steel according to claim 6, wherein the steel has an elongation after fracture in a direction at a right angle to a rolling direction of 28% or more, an average Lankford value of 0.70 or more, and a ridging height of 2.5 μm or less.
 10. A method for manufacturing the ferritic stainless steel according to claim 2, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 11. A method for manufacturing the ferritic stainless steel according to claim 3, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 12. A method for manufacturing the ferritic stainless steel according to claim 4, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 13. A method for manufacturing the ferritic stainless steel according to claim 6, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 14. A method for manufacturing the ferritic stainless steel according to claim 7, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 15. A method for manufacturing the ferritic stainless steel according to claim 8, the method comprising: performing hot rolling on a steel slab to form a hot-rolled steel sheet; then performing annealing in which the hot-rolled steel sheet is held at a temperature in a range of 900° C. or higher and 1050° C. or lower for 5 seconds or more and 15 minutes or less; then performing cold rolling; and then performing annealing in which the cold-rolled steel sheet is held at a temperature in a range of 800° C. or higher and 950° C. or lower for 5 seconds or more and 5 minutes or less.
 16. A method for manufacturing the ferritic stainless steel according to claim 9, the method comprising: 